Articles are laser marked in order to ensure product safety and authenticity. For example, packaging material of pharmaceuticals is laser marked to enable a consumer to know the genuineness of a product. Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various papers or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such articles and security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
Today, laser marking employed in the manufacture of security documents consists solely of a “black” laser marking method via the carbonization of a polymer, usually polycarbonate as disclosed in e.g. EP 2181858 A (AGFA)). Recently, there has been considerable interest in being able to produce colour images through laser marking.
US 2005001419 (DIGIMARK) discloses a colour laser engraving method and a security document including an opaque surface layer and one or more coloured sub-layers. A laser provides openings in the surface layer to expose the colour of the sub-layer thereby creating colour images and text. Such a security document can be falsified by providing additional holes or filling up existing holes on the security document.
U.S. Pat. No. 7,158,145 (ORGA SYSTEMS) discloses a three-wavelength system (440, 532 and 660 nm) for applying coloured information to a document by means of wavelength-selective bleaching of chromophoric particles in a layer close to the surface. Although a bleached colour cannot be restored, modification of the coloured information remains possible by extra bleaching.
U.S. Pat. No. 4,720,449 (POLAROID) discloses a thermal imaging method for producing colour images on a support carrying at least one layer of a colourless compound, such as di- or triarylmethane, by conversion of electromagnetic radiation into heat. The laser beam may have different wavelengths in a range above 700 nm with at least about 60 nm apart so that each imaging layer having a different infrared absorber may be exposed separately to convert a colourless triarylmethane compound into a coloured form. There is however no disclosure how to prevent modification of the colour image by additional laser marking, which is also the case for similar imaging methods disclosed in WO 2009/140083 (3M) and U.S. Pat. No. 4,663,518 (POLAROID)
U.S. Pat. No. 5,219,703 (KODAK) discloses a laser-induced thermal dye transfer imaging method wherein the infrared dye absorbs laser radiation and converts it into heat which vaporizes dyes in a dye-donor element which are transferred to a dye-receiver element. After transfer, the infrared dyes which cause an undesirable visual light absorption in the dye-receiver element are bleached by an acid-photogenerating compound upon subsequent exposure to infrared or ultraviolet radiation. The infrared dyes in the thermal dye transfer material of EP 675003 A (3M) are bleached using a thermal bleaching agent upon exposure to infrared radiation.
A solution to hinder or prevent modification of the colour image by additional laser marking would be to use the bleaching of the infrared dye as applied in the field of thermal dye transfer imaging. However, bleaching with UV radiation generally results in colour formation from the colourless dye. Radiation of an infrared laser results also in additional colour formation, while a long infrared exposure of low intensity not resulting in colour formation is economically not viable.
Therefore, it would be desirable to have a secure colour laser marking system for producing security documents wherein the laser marked colour image cannot be modified by additional colour laser marking.